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In this issue of the Journal of Clinical Pathology, Dr. Nishikori discusses the challenges in distinguishing between the idiopathic plasmacytic lymphadenopathy (IPL) type of idiopathic multicentric Castleman disease (iMCD) and IgG4-related lymphadenopathy.1 They highlight the significant overlap in clinical, immunological and serological features between the two diseases. Notably, no differences in serum IgG4 levels were observed. However, iMCD-IPL patients tend to be older and exhibit higher serum IgG, IgA, IgM, IgE and acute phase reactants such as C-reactive protein (CRP) while showing lower haemoglobin and albumin levels. However, the combination of serum IgG4/IgG ratio (<19%), serum IgG (>5381 mg/dL) and elevated CRP (>6 mg/dL) may aid in the distinction between iMCD-IPL and IgG4-related disease (IgG4-RD) (table 1).